Most of the optical materials such as optical fibers, optical connectors or other optical components are subject to a birefringence when an optical path for an incident light beam is deviated from optical axes of the materials. Namely, an optical beam is polarized when passing through or reflected from such birefringent materials. Thus, for example, when a circularly polarized light beam is emitted to a birefringent material and is transmitted through the birefringent material or reflected by the birefringent material, the circular polarization of the light beam is affected to be an elliptically polarized or linearly polarized light beam. As a result of such birefringence, the light beam suffers a loss of power, i.e., subject to the polarization dependent loss.
To determine the polarization characteristics or the amount of the polarization dependent loss of the optical material by measuring an azimuth (polarization angle) and an ellipticity, the conventional technologies shown in FIG. 7 have been generally used. FIG. 7A is a block diagram showing a conventional measuring method using a Soleil-Babinet compensator 14, FIG. 7B is a block diagram showing a conventional measuring method using a rotating analyzer 17, and FIG. 7C is a block diagram showing a conventional measuring method based on a power splitting process.
In FIG. 7A, a light source 11 emits a coherent light beam 5 which passes through a polarizer 13, a Soleil-Babinet compensator 14, and a device under test (DUT) 4. The DUT 4 in this case is a type of optical material or device through which the light beam 5 can transmit. The light beam from the DUT 4 is received by an analyzer 16 and a photo detector 18.
The polarizer 13 is to separate a certain polarization in the light beam from the other. The analyzer 16 has the same structure of polarizer 13 and is used for detecting the polarization of the incoming light beam. The Soleil-Babinet compensator 14 is to adjust the degree of birefringence in the light beam passing therethrough by changing a mechanical position of wedges forming the compensator. The photo detector 18 is typically a photo diode to detect the optical signal and converts the detected optical signal to a corresponding electric signal.
In the arrangement of FIG. 7A, prior to the measurement of the light beam from the DUT 4, the polarizer 13 and the analyzer 16 are first adjusted without the DUT 4 and the Soleil-Babinet compensator 14. The adjustment is made so that the optical axes of the polarizer 13 and the analyzer 16 are perpendicular with each other. In this situation, when the coherent light beam 5 is emitted, the light beam 5 is prohibited to reach the photo detector 18 because of the optical axes. Thus, the output electric current produced by the photo detector 18 is the smallest amount. When inserting the DUT 4 between the polarizer 13 and the analyzer 16 as shown in FIG. 7A, a small amount of light beam passes through the analyzer 16 and reaches the photo detector 18 since the light beam 5 is elliptically polarized by the DUT 4.
Then the Soleil-Babinet compensator 14 is inserted between the polarizer 13 and the DUT 4, and the wedge of the compensator 14 is adjusted by, for example, a micrometer head (not shown), until a point will be found where the light beam is again prohibited to reach the photo detector 18. In this point, the output electric current of the photo detector 18 becomes the minimum amount as above.
In this situation, the azimuth and ellipticity of the polarization caused by the DUT 4 can be calculated base on the reading of the micrometer which has driven the Soleil-Babinet compensator 14. Further, based on the azimuth and ellipticity thus obtained, a polarization dependent loss can also be calculated. In the example of foregoing process, the minimum point of input power to the photo detector 18 is detected. In a similar manner, it is also possible to utilize the maximum point of power as a target point by arranging the polarizer 13 and the analyzer 16 in the same optical axis.
In the example of FIG. 7B, a light source 11 provides a coherent light beam 5 which passes through a circular polarization converter 12, and a device under test (DUT) 4. The light beam from the DUT 4 is received by a rotating analyzer 17 and a photo detector 18.
The circular polarization converter 12 is formed of a polarizer 13 and a 1/4 wavelength (.lambda./4) plate 15. The circular polarization converter 12 is to convert an incoming light beam 5 to a circularly polarized light beam 8. The rotating analyzer 17 is basically the same as the analyzer 16 of FIG. 7A, however it mechanically rotates by a predetermined constant rotation rate. The photo detector 18 is typically a photo diode to detect the optical signal and converts the detected optical signal to a corresponding electric signal.
The light beam 5 emitted from the light source 11 is converted to a circularly polarized light beam 8 by the circular polarization converter 12. The DUT 4 is irradiated by the circularly polarized light beam 8 and the resulted light beam 9 from the DUT 4 may be elliptically polarized because of the birefringence of the DUT 4. The transmitted light beam 9 reaches the photo detector 18 through the rotating analyzer 17.
Since the rotating analyzer 17 rotates in the constant rate, the light beam detected by the photo detector 18 periodically changes its intensity. In other words, the light beam passing through the rotating analyzer 17 is converted to an alternating signal having the high intensity light beam and the low intensity light beam. This is because the light beam is intensified when the optical axes of the DUT 4 and the rotating analyzer 17 are aligned while the light beam is weakened when the optical axes of the DUT 4 and the rotating analyzer 17 intersect at 90 degrees. Thus, the azimuth and ellipticity caused by the DUT 4 can be calculated based on the rotation angle of the rotating analyzer 17. Further, based on the azimuth and ellipticity thus obtained, a polarization dependent loss can also be calculated.
In the example of FIG. 7C, like the example of FIG. 7B, a light source 11 provides a coherent light beam 5 which passes through a circular polarization converter 12, and a device under test (DUT) 4. Unlike the example of FIG. 7B, the light beam from the DUT 4 is received by a beam splitter 19, Wollaston prisms 201 and 202, polarization plates 221, 222 and 223, a .lambda./4 plate 152, and photo detectors 231-234 as shown in FIG. 7C.
The circular polarization converter 12 is formed of a polarizer 13 and a .lambda./4 plate 15.sub.1. The circular polarization converter 12 is to convert an incoming input light beam 5 to a circularly polarized light beam 8. The beam splitter 19 splits the circularly polarized light beam 8 into two light beams, one goes straight forward and the other goes upper direction in FIG. 7C. The Wollaston prisms 201 and 202 are birefringent polarization prisms which separate the light beam passing therethrough into an ordinary beam and an extraordinary beam. The optical axes of the ordinary beam and the extraordinary beam from the Wollaston prism are perpendicular with each other.
Thus, by properly aligning the optical axes of the Wollaston prisms 201 and 202, and by passing through the .lambda./4 plate 152 and the polarization plates 221-223, each of the four polarization components can be detected by the corresponding photo detectors 231-234. By comparing the intensities of the four polarization components detected by the photo detectors 231-234, the polarization state such as the azimuth and ellipticity caused by the DUT 4 can be calculated. Further, based on the azimuth and ellipticity thus obtained, a polarization dependent loss of the DUT 4 can also be calculated.
In the foregoing conventional measuring methods, for example in FIG. 7A using the Soleil-Babinet compensator or in FIG. 7B using the rotating analyzer, a relatively large response time is required since these methods involve the mechanically moving parts. Thus, in the examples of FIGS. 7A and 7B, it is not possible to realize a high speed measurement. In the example of FIG. 7C, a detection sensitivity is low, since the light signal from the device under test is divided into four light beams. Thus, it is difficult to improve the measurement accuracy or sensitivity in the example of FIG. 7C.